This proposal addresses itself to the relationship between arterial blood pressure, intrarenal hemodynamics and urine volume and composition. Specifically, the in situ pump-perfused kidney will be used to evaluate changes in perfusion pressure on the intrarenal distribution of radioactive microspheres and concommittant urine volume and composition. Effects of pressure from below the autoregulatory range to 160 mmHg will be used to determine if there is a redistribution of intrarenal blood flow which might account for the changes in urine volume and composition observed. In addition, various methods will be used in this same model to attempt to uncouple redistribution of intrarenal blood flow from changes in excretion. This proposal presents a unique explanation to explain autoregulation of renal blood flow and glomerular filtration rate while urine flow is not autoregulated.